Benzamide and 3-aminobenzamide, potent inhibitors of poly (ADP-ribose) polymerase, increased MMS or MNNG-induced DNA strand breaks and significantly retarded the rejoining of strand breaks in CHO cells. These inhibitors also synergistically enhanced the frequencies of unscheduled DNA synthesis (UDS) and sister chromatid exchanges (SCE) induced by MNNG. Amounts of UDS and strand breaks induced by the combined treatment with MMS and MNNG were found to be additive but these amount were enhanced in the presence of inhibitors. These results suggest that poly (ADP-ribose) functions a regulatory role in the repair of DNA damage by virtue of stabilizing chromatin structure whenever strand breaks occur in DNA.